Spread Spectrum Transmission And Mixers In Radio Systems

Why is spread spectrum transmission used?

1. Why is spread spectrum transmission used? Comparison between two common methods used for spread spectrum transmission?

2. Different components of a radio system. What are different mixers? Explain different types of Mixers with diagram? 

Ans1. Spread spectrum refers to the form of wireless communication where the transmitted signal’s frequency is varied deliberately, which ultimately results in the formation of a bandwidth which is much greater than the bandwidth which the signal would be having when the frequency was not varied. The signal transmitted occupies a much higher bandwidth than the systems which are used in the conventional modulation methods (Torrieri, 2015). The spreading of the band is achieved my selecting waveforms of appropriate bandwidth which generally consists of waveforms with wide bandwidth. One of the most popular way is by multiplying the data signals of the user with a fast code sequence and this is mostly independent of the data message which has been already transmitted (Rappaport et al., 2013). For this case the radio portion that is shared by the users is same but the code used is different so as to distinguish the transmissions made by them.

There exists mainly methods for implementation of multiple access and they are namely FDMA or Frequency Division Multiple access, TDMA or Time Division Multiple Access and lastly CDMA or Code Division Multiple Access.

Time Division Multiple Access is the method which generally comes under the process of modulation. This method is associated with providing of users at a larger rate so as to access the single radio frequency channel or RF channel without the involvement of any type of interference and is associated with allocating each user with an independent time slot (Sha, Qiu & Mei, 2012). In this method different types of users communicate with each other which means listening and speaking in accordance to the time slots that has been allocate to them. It is mainly used in the digital cellular telephones and is associated with dividing each and every cellular channels into three time slots for the purpose of increase the data that is to be carried. TDMA was firstly identified as the standard in the EIA/TIA Interim Standard 54 or IS-54. This later evolved to Interim Standard-136 or IS-136 and is used by US  standards for the TDMA for cellular as well as the personal communication services spectrum which is having an frequency of around 850MHz and 1.9GHz.the TDMA is also used for the DECT or Digital Enhances Cordless Telecommunication. The mobile stations radio components are allowed to listen and broadcast by the TDMA only in the time slot assigned to them. Along with this during rest of the time the mobile stations might apply the network measurements by the detection of the different transmitters that surrounds it having different frequencies.

Comparison between two common methods used for spread spectrum transmission

Code Division Multiple Access generally refers to the different types of protocols which are generally used in the second generation or 2G and the third generation or 3G wireless communication. According to the term it can be stated that is one of the form of multiplexing and is generally associated with allowing several signals to occupy a single channel responsible for transmission. This also optimizes the use of the bandwidth which is available. This technology is generally used in the ultrahigh frequencies cellular telephone system which has a frequency range of around 899MHz and 1.9 GHz.  Conversion of the analog to digital signal is also employed by the CDMA (Kaddoum, Richardson & Gagnon, 2013). This is associated with sharing of the channels for the purpose of transmitting information of different type which is generally having a definite RF bandwidth which is mainly associated with the other users instantaneously and also with the interference of each other. This is initially responsible for providing flexibility at larger amounts and overall system capacity which is high with respect to the Cellular systems. Each digital packet in the CDMA is sent along with a unique key and the responsible receiver which possess the key to that code is capable of picking up those signals quickly and then demodulate that for this reason a special type of coding scheme is followed in order to allow numerous users to be multiplexed over same channels.

Ans2: Different types of radio components are available for appropriate platform installation (Sadegh Amiri et al., 2013). The different components of a radio spectrum are listed below:

• Filter and Multi-coupler
• Power amplifiers
• Antenna tuning units
• Remote control units
• Power suppliers
• Audio units
• System racks

In a radio system a mixer or a frequency mixer can be considered as a non-liner electrical circuit that is associated with the creation of new frequencies by making use of two different signals which are applied to the mixer. Two signals are applied to the mixers so as to produce a new signal with respect to the sum and difference of the two original frequencies. A frequency mixer or a mixer can produce other frequency components as well. There are two types of mixers and this includes the Diode mixers and switching mixers (Hotokezaka & Piran, 2015). Mixers are also widely used for the purpose of shifting signals from one range to the other, this process is also known as heterodyning. This is done for the transmissions convenience or for further processing of the signals. The devices which are having a non-liner characteristic can be considered as a mixer.

Different components of a radio system

Passive mixers are those mixers which generally makes use of a single diode ora numver of diode and along with this the passive mixers mainly depends on the non-liner relation that exists between the voltage and the current so as to supply an element having a multiplying nature. The passive mixers produces an output which is at all times lower in value than the value of the input signals.

Active mixers are something which makes use of an amplifying device so as to increase the strength of the signal product. This type of mixers are having an improved isolation which exists between the different ports. Along with this this there might also exist high noise and more consumption of power (Hotokezaka & Piran, 2015). The active mixers might also be less tolerant to the overload.

Mixers can also be classified according to their topology and they are listed below:

Unbalance mixers: This type of mixers are associated with allowing both of the input signals to pass through and appear as component in the output.

Single balanced mixers: This type of mixers are arranged with on eof the inputs and are applied to a balanced circuit in order to suppress either the local oscillator or the signal input at the output but in both.

Double balanced mixer: This type of mixer consists of both the inputs which are applied to the differential circuits so as to make sure that at the output there is no appearance of the input signals. Only the product signal should appear at the output (Lin, Mak & Martins, 2014). This type of mixers are more complex and generally requires higher drive levels tan the unbalanced and the signal balanced designs.

A diode is generally used for the purpose of creating a simple unbalanced mixture. And this type of mixtures are associated with producing the original frequency along with the sum and difference of the frequency. One of the significant feature of the diode is its non-linearity which means the response of this not proportional to the input. Along with this the diode is also not associated with the reproducing of the frequencies responsible for its driving voltage in the current which is passing through the diode. For this reason the manipulation is possible as desired by the user.

Switching is one of another way in which the mixer operates. This operation is associated with a smaller amount of input signal which is generally passed in inverted or non-inverted form and this happens according the phase of the local oscillator (Lin, Mak & Martins, 2014). According to the mathematical term the switching mixer is almost similar to the multiplying mixer and this is because in the switching mixer signal function is used instead of the LO sine wave. Switching is considered to be the typical form of the normal operations that is done by the double balanced mixer which is having a local oscillator so as to drive at a considerable higher speed than the amplitude of the signal. The most essential advantage of the switching mixer is that it is capable of achieving a noise figure of lower range and conversion gain of higher range. This happens because of the reason that the switching diode or the transistors generally act as a small resistor or like a large resistor (Myslivets et al., 2012).  For both the cases a minimal amount of noise is added. From the perspective of a circuit it can be stated that many of the multiplying mixers can be used as a switching mixer and this can be done by only increasing the LO amplitude.

There is another type of mixer known as the harmonic mixer and the subharmonic mixer which is considered to be a circuit associated with charging one signal frequency to the other. The harmonic and subharmonic mixers is associated with the formation of the sum and difference frequencies which is the harmonic multiple of any one of the inputs. Whereas the normal mixers consists of two input signals and one output signal.

References:

Hotokezaka, K., & Piran, T. (2015). Mass ejection from neutron star mergers: different components and expected radio signals. Monthly Notices of the Royal Astronomical Society, 450(2), 1430-1440.

Kaddoum, G., Richardson, F. D., & Gagnon, F. (2013). Design and analysis of a multi-carrier differential chaos shift keying communication system. IEEE Transactions on Communications, 61(8), 3281-3291.

Lin, F., Mak, P. I., & Martins, R. (2014, February). 3.9 An RF-to-BB current-reuse wideband receiver with parallel N-path active/passive mixers and a single-MOS pole-zero LPF. In Solid-State Circuits Conference Digest of Technical Papers (ISSCC), 2014 IEEE International (pp. 74-75). IEEE.

Myslivets, E., Kuo, B. P., Alic, N., & Radic, S. (2012). Generation of wideband frequency combs by continuous-wave seeding of multistage mixers with synthesized dispersion. Optics express, 20(3), 3331-3344.

Rappaport, T. S., Sun, S., Mayzus, R., Zhao, H., Azar, Y., Wang, K., ... & Gutierrez, F. (2013). Millimeter wave mobile communications for 5G cellular: It will work!. IEEE access, 1, 335-349.

Sadegh Amiri, I., Nikmaram, M., Shahidinejad, A., & Ali, J. (2013). Generation of potential wells used for quantum codes transmission via a TDMA network communication system. Security and Communication Networks, 6(11), 1301-1309.

Sha, X. J., Qiu, X., & Mei, L. (2012). Hybrid carrier CDMA communication system based on weighted-type fractional Fourier transform. IEEE Communications Letters, 16(4), 432-435.